TIME-DEPENDENT FLOW IN ARRAYS OF TIMBER BOARDS - FLOW VISUALIZATION, MASS-TRANSFER MEASUREMENTS AND NUMERICAL-SIMULATION

A numerical simulation of the airflow patterns around an array of boards in a timber kiln has been modified to include the prediction of time-dependent flows, and the predictions of the simulation have been compared with airflow patterns recorded in a previous flow visualization study and experimental measurements of the mass-transfer coefficients along the boards. The numerical simulation solves the two-dimensional Navier-Stokes equations on a finite-difference grid. The simulation predicts that the enhancement of the asymptotic values of the mass-transfer coefficients above those predicted by conventional flat-plate correlations is due to the effects of the gaps between the boards in generating additional turbulence throughout the array of boards. The enhancements at the leading edges of the boards are also predicted by the simulation to be a consequence of the time-dependent nature of the flow. Periodic oscillations in the flow patterns around the gaps with a period of 1-7 s were observed in the flow visualization study, in agreement with the period of 2 s predicted by this simulation. The variation in the experimentally observed time period may be due to the sensitivity of the vortex-shedding phenomenon to small differences in alignment within the geometry between experiments. The enhancement in the mass-transfer coefficients at the leading edge of the first board is not predicted well. because the treatment of the flow near the wall requires improvement in the simulation.